DESCRIPTION: (Adapted from the application): Enkephalins inhibit release of autonomic transmitters, thus, decreasing responses to nerve stimulation. For instance, enkephalins reduce vagally-mediated bradycardia. Conversely, adrenergic stimulation to cardiomyocytes causes increased enkephalin production. Increased adrenergic stimulation is seen in hypertension. This led the investigators to speculate that the decreased cardiac responsiveness to vagal nerve stimulation seen in hypertension may be linked to cardiac opioid peptides. Indeed, preliminary data show increased cardiac enkephalin and catecholamine content in hypertensive dogs. The investigators hypothesize that 1) hypertension-induced increases in sympathetic activity decrease enkephalin degradation leading to increased cardiac enkephalins; and 2) increased enkephalins cause decreased responsiveness to autonomic stimulation and increased cardiac catecholamine content due to decreased cardiac catecholamine release. Experiments will be conducted in vivo with chronically instrumented hypertensive and control dogs with and without beta-adrenergic receptor blockade. Cardiac enkephalin degrading activity will be measured in vitro. The experiments are designed to answer: Are increases in sympathetic activity responsible for increased enkephalins? Are cardiac enkephalins released in vivo? Do opioids depress cardiac responses to autonomic stimulation? Does hypertension change cardiac proenkephalin processing or enkephalin degradation? 1) Baroreflex challenges will be performed in conscious dogs before and during development of renovascular hypertension or the sham procedure. Decreased arterial pressure by nitroprusside stimulates sympathetic and depresses vagal outflows. Increased arterial pressure by phenylephrine inhibits sympathetic and stimulates vagal outflows. Cardiac enkephalin and catecholamine plasma spillover and cardiovascular function will be measured. In addition, spillover and cardiac function will be measured with direct stimulation of the vagus and sympathetic nerves in anesthetized animals before tissue collection in all groups. 2) Opioid peptide influence on baroreflex function will be tested by repeating baroreflex challenges in the presence of the non-selective opioid receptor antagonist, diprenorphine. 3) Measurement of tissue enkephalinase activity, proenkephalin products and catecholamine content in hypertensive vs. sham control animals will be made. Cardiac specificity will be determined by examining other tissues (adrenal gland, kidney, lung, skeletal muscle). Hypertension remains a serious cardiovascular problem in the population. It is suggested that the results will advance knowledge and may improve therapeutic use of opiates in patients with cardiac pathology.